Varactors are electronic devices that have a capacitance that is controlled by a suitable voltage or current bias. Varactors are typically employed, for example, in so-called voltage controlled oscillators (VCOs) where a frequency of an oscillator is controlled by an applied current or voltage. In such instances, the VCOs are used when a variable frequency is required, or when a signal needs to be synchronized to a reference signal.
In radio communication devices, such as mobile/cellular phones, VCOs are typically employed in phase locked loop circuits to generate suitable signals including: generation of a reference signal that is synchronized with a signal received by a radio receiver, modulation/demodulation operations and frequency synthesis.
Numerous varactors have been developed and are successfully employed in integrated circuit technologies. For example, it is known to employ pn-diodes, Schottky diodes or MOS-diodes as a varactor in bipolar, CMOS and BiCMOS technologies. In the article to R. A. Moline, et al., entitled “Ion-implanted Hyperabrupt Junction Voltage Variable Capacitors” IEEE Trans. Electron. Device, ED-19, pp267f, 1972, varactors comprising pn-diodes are described. U.S. Pat. No. 3,638,300 to Foxhall, et al.; U.S. Pat. No. 4,226,648 to Goodwin, et al.; U.S. Pat. No. 4,827,319 to Pavlidis, et al; and U.S. Pat. No. 5,557,140 to Nguyen, et al. describe other types of variable capacitor (i.e., varactor) diodes that include hyper-abrupt ion-implanted junctions. The term ‘hyper-abrupt’ denotes that the implant has a doping profile that is in contact with the wall of the adjacent extrinsic base region. U.S. Pat. No. 4,973,922 to Embree, et al.; U.S. Pat. No. 5,965,912 to Stolfa, et al; and U.S. Pat. No. 6,100,770 to Litwin, et al., on the other hand, describe MOS-diodes that are employed as varactors.
The integration of varactors depends on the capability of the integrated circuit technology. An overview of integrated circuit devices for high RF applications in BiCMOS technology is described, for example, in J. N. Burghartz, et al. “Integrated RF and Microwave Components in BiCMOS Technology”, IEEE Trans. Electron Devices, Col. 43, pp1559, September 1996. As is stated therein, varactors are not a part of the standard BiCMOS device set. Instead, it is proposed to employ a collector-base junction of a bipolar transistor as a varactor.
In order to use a device as a varactor, the device must satisfy one or more, preferably two or more, of the following criteria: (1) tunability (i.e., ratio of maximum capacitance to minimum capacitance) must be high (on the order of about 3 or greater); (2) Quality factor Q must be high (on the order of about 20 or greater); and (3) the device must exhibit linearity.
Many of the known prior art varactors do not meet the above criteria. For example, traditional base-collector junction varactors rely on the NPN base-collector profile, which is not optimized for varactor tunability. In the case of hyper-abrupt base-collector junction varactors, where the doping profile of the implant is located at the “wall” of the extrinsic base region, the device lacks linearity. With traditional MOS varactors, the tunability is high; however, an even higher tunability is often required.
In view of the above-mentioned drawbacks with prior art varactors, there is a continued need for providing new and improved varactors, which satisfy the above criteria and that can be integrated with CMOS and BiCMOS devices.
Moreover, in conventional MOS varactor designs it is difficult to electrically isolate the MOS varactor from the bulk substrate. The lack of sufficient electrical isolation results in a device that has high parasitics, e.g., noise. As such, there is also a need to provide a varactor design that has sufficient electrical isolation thereby reducing noise in the device.